You mean with a magic ox you do not need fertilizer, and weed and bug management not to mention all the types of plant fungus.
I am assuming that since the field is plowed (the whole point), no RoundUp™ or diesel fuel is needed.. That starts the draft animal farm off with a huge advantage per acre. After the crop is planted, the costs are exactly the same.
We have a rather large organic farmer near her (200 acres or so). No chemicals, and they have chicken houses on skids that they move around the farm to provide the fertilizer.
Last year, we had a lot of rain right after planting, so the weeds overtook the crops before they could even get the sprayers in the field.
The no-herbicide guy, ended up plowing under his entire crop of soybeans that was overrun by the weeds before he could get a cultivator in the field.